Presses are used to consolidate paper and panel products. Examples of this consolidation are the formation of a pulp mat from a pulp slurry, the formation of paper from wood pulp or other fibrous material, or the formation of a panel product from wood particles or flakes. Compressive forces act on and consolidate the material as it passes through the nip formed by a pair of rolls. The greater the compressive force the greater the consolidation.
The compressive forces at the nip perform another function in the formation of paper--the removal of water from the web.
The compressive forces acting on a web in the nip between the two rolls is of short duration. The time that the compressive force may act on the web may be extended by the use of a belt press. In a belt press a belt is wrapped around a section of the periphery of a drum and exerts a compressive force on a web passing between the belt and drum. Tension in the belt is translated into a compressive force on the web and drum. Belt presses are used both for paper and for panel products. Gottwald et al, U.S. Pat. Nos. 3,110,612 and 3,354,035 and Haigh, U.S. Pat. No. 3,319,352 are exemplary of belt presses for paper. Gersbeck et al, U.S. Pat. No. 3,891,376, Brinkmann et al, U.S. Pat. No. 3,938,927 and Gerhardt et al, U.S. Pat. No. 4,457,683 are exemplary of belt presses for panel products.
FIGS. 1-10 illustrate compressive forces from belts and nips acting on a web. These figures also illustrate the forces that are being passed to the frame of the apparatus. In the illustrations of the compressive forces on the web in both the background section and the detailed description section a number of parameters are held constant. These are:
(a) The belt tension (T),
(b) The belt materials,
(c) The conditions in the nip, e.g., web thickness, roll covering, etc.,
(d) The constant surface temperature of the drum, and
(e) The forces due the rotational drive forces and the component weight.
In addition, relative roller diameter and belt angles are arbitarily selected to simplify analysis. The diameter and belt angle options are infinite but the arbitrary selection will not greatly distort the illustration. Also, supplemental nip forces mentioned often in the art are not taken into account in the examples.
The only variable being analyzed is the total compressive force (TCF) produced by belt tension or directly by belt tensioning forces available to compress the web being processed. These forces are expressed as a multiple of belt tension T. Both and TCF may be expressed in suitable force units such as pounds.
There are three categories of compressive force acting on the web. These are:
(1) The total compressive force radial to the central drum caused by that portion of the belt resting directly on the central drum and due to tension in that portion of the belt only. This quantity is equal to:
T2.pi. (% of central drum circumference contacted/100)
(2) The nip force of each of the belt tension rollers when these rollers make a nip with the central drum.
(3) The nip force of each of the belt carrying idler rollers other than the tension rollers upon the central drum when these rollers make a nip with the central drum. The force is created by the belt tension only.
FIGS. 1-10 are representative of prior art drum and belt presses.
FIG. 1 illustrates the configuration shown in FIG. 1 of Gottwald et al, U.S. Pat. Nos. 3,110,612 and 3,354,035. FIG. 2 illustrates the configuration described in line 25 of column 4 of Gottwald et al, U.S. Pat. No. 3,110,612. In both of these figures the total compressive force is created solely by the belt resting on the central drum. There is no nip force on the central drum.
In FIG. 1 the belt 3 circumferentially contacts 180.degree. or 50% of the surface of central drum 4. The tension T on the belt is provided by the two tensioning rollers 5 and 6. The idler roller 7 holds the inner and outer courses of belt 3 apart. The web 8 is guided around the central drum 4 and pressed against the central drum 4 by the belt 3. The total compressive force on the central drum 4 and web 8 is equal to 3.14 T. The tensioning rollers 5 and 6 are attached to a frame and the tension of approximately 2 T is transferred to the frame from each roller. In addition, there is an axial bending force of 2 T on the central shaft of the central drum 4. There is also an axial bending force of approximately 2 T on each of the central shafts of tensioning rollers 5 and 6 and idler roller 7. The central drum 4, the tensioning rollers 5 and 6, and the idler roller 7 are all attached to the frame and the forces upon them are transmitted to the frame. Neither the tensioning rollers 5 and 6 nor the idler roller 7 form a nip with the central drum 4.
In FIG. 2 the belt 3a circumferentially contacts 270.degree. or 75% of the surface of central drum 4a. The tensioning rollers are 5a and 6a and the idler rollers are 7a, 9 and 10. The web 8a is guided around the central drum 4a and pressed against the central drum 4a by the belt 3a. The total compressive force acting on the central drum 4a and the web 8a is 4.7 T. Again, there is an axial bending force applied to the central shaft of central drum 4a and an axial bending force applied to each of the tensioning rollers 5a and 6a and idler rollers 7a, 9 and 10. These forces are passed on to the frame for the apparatus and the frame must be strong enough to carry them.
Haigh, U.S. Pat. No. 3,319,352; Gersbeck et al, U.S. Pat. No. 3,891,376; and Brinkmann et al, U.S. Pat. No. 3,938,927 are exemplary of configurations in which one or more idler nip rolls are used.
In each of the following examples the total compressive force caused by the belt on the central drum will be the same as those calculated for FIGS. 1 and 2-3.14 T at 50% circumferential contact between the central drum and the belt.
FIG. 3 illustrates a configuration in which there is one idler nip roll. The belt 3b and the web 8b circumferentially contacts 50% of the surface of the central drum 4b. The tensioning rollers are 5b and 6b. An idler nip roller 11 is within the belt 3b and forced toward central drum 4b by the outer course 3b' of belt 3b and forms a nip 12 with the central drum 4b. The web 8b is guided around and pressed against the central drum 4b by the inner course 3b" of belt 3b. The idler roller 11 also compresses the belt 3b and web 8b in the nip 12. The compressive force in nip 12 is 2 T. The total compressive forces--idler roller nip force and belt force--are 5.4 T. There will also be 4 T of axial bending force acting upon the central drum 4b and 2 T of axial bending force acting on each of the tensioning rollers 5b and 6b. These forces are transferred to the frame of the apparatus.
FIG. 4 illustrates a configuration in which there are two idler nip rollers. The belt 3c and web 8c train around 50% of the surface area of central drum 4c and the belt 3c is held in tension by tensioning rollers 5c and 6c. A pair of idler nip rollers 13 and 14 are within belt 3c and are placed at a 45.degree. angle to the axis of central drum 4c. The idler nip rollers 13 and 14 are forced toward central drum 4c by the outer course 3c' of belt 3c and form nips 15 and 16 with the central drum 4c. The web 8c is guided around and pressed against the central drum 4c by the inner course 3c" of belt 3c. A vector analysis of the forces acting upon each of the idler nip rollers is shown in FIG. 5. Roller 13 is illustrated. The resultant compressive force is 1.4 T in each of the nips 15 and 16. The total compressive forces acting on web 8c--the belt compressive force and the nip compressive force--are 5.94 T. The axial bending forces of 2 T on each of the tensioning rollers 5c and 6c, and 4 T on central drum 4c are transferred to the frame.
FIG. 6 illustrates the system shown in FIG. 4 and the average pressures acting on the central drum 4c and the web 8c at various locations around the drum. For purposes of illustration the following parameters were chosen--1000 pounds per lineal inch (pli) belt tension and a 50 inch drum diameter. This results in a compressive force from the belt of 40 pounds per square inch (psi). An average nip pressure of 500 psi is assumed. The belt pressure is continuous over 50% of the drum surface and the nip pressure is discontinuous as shown.
FIG. 7 illustrates a configuration in which there are three idler nip rollers, central idler nip roller 17 and side idler nip rollers 19 and 20. The idler nip rollers 17, 19 and 20 are forced toward central drum 4d by the outer course 3d' of belt 3d to form nips 18, 21 and 22 with the central drum 4d. The web 8d is guided around and pressed against central drum 4d by the inner course 3d" of belt 3d. The forces acting on central idler roller 17 are the same as those shown for idler roller 13 in FIG. 5. The compressive force acting on the web 8d in the nip 18 is 1.4 T. A vector diagram of forces acting on side idler rollers 19 and 20 is shown in FIG. 7. The compressive force acting on the web 8d in each of the nips 21 and 22 is 0.7 T. The total compressive forces acting on the web 8d are 5.94 T. The axial bending forces of 2 T on each of the tensioning rollers 5d and 6d, 3.414 T on central drum 4d and 0.29 T on each of the side idler rollers 19 and 20 are transferred to the frame.
FIG. 8 illustrates a configuration in which there are four idler nip rollers, central idler nip rollers 23 and 24 and side idler nip rollers 27 and 28. The idler nip rollers 23, 24, 27 and 28 are forced toward central drum 4e by the outer course 3e' of belt 3e to form nips 25, 26, 29 and 30 with the central drum 4e. The web 8e is guided around and compressed against central drum 4e by the inner course 3e" of belt 3e. A vector diagram of forces acting on central idler nip rollers 24 and 25 is shown in FIG. 9. Central idler nip roller 24 is illustrated. The compressive force acting on the web 8e in each of the nips 25 and 26 is T. The compressive force acting on the web 8e in each of the nips 29 and 30 is shown in FIG. 8. It is 0.5 T. The total compressive forces acting on the web 8e during its travel around the central drum 4e are 6.14 T. Again the axial bending forces acting on the central drum 4e, the tensioning rollers 5e and 6e, and the idler nip rollers 23, 24, 27 and 28 are transferred to the frame.
FIG. 10 illustrates a configuration in which there is a large number of idler nip rollers. In this configuration the idler nip rollers 30 extend throughout the area of belt and web contact with the central drum 4f. The idler nip rollers 31 are forced toward central drum 4f by the outer course 3f' of belt 3f to form nips 31 with the central drum 4f. Two belt and web guide rollers 32 and 33 are added. The web 8f is guided around and compressed against central drum 4f by the inner course 3f" of belt 3f. In this configuration the total compressive forces acting on the web through the nips of the idler nip rollers are approximately equal to the total compressive forces from the belt. The total compressive forces acting on the web will be 6.28 T. The axial bending forces on the tensioning rollers 5f and 6f, and the central drum 4f are transmitted to the frame.
In each of the above belt loop configurations, forces from the belt and roller system are carried by the frame. In each of these configurations, the central drum must be mounted on the frame and the unbalanced compressive force on the shaft of the central drum, and on the shafts of the tensioning and some idler rollers is passed to the frame. The unbalanced compressive forces acting on the shafts and on the frame range from 1.57 T to 4 T. The central drum is heavy and the shell is thick in order to absorb these forces with allowable bending stress.
If the press is used as a dryer, then the drum will usually be heated. U.S. Pat. No. 4,324,613 discloses a pair of nip rolls for consolidating and drying paper in which one of the rolls is a heated drum. In belt presses, the belt may wrap around a heated drum. The Gottwald et al, Haigh, Gersbeck et al, Brinkmann and Gerhardt et al patents disclose a heated central drum. In conventional practice, the thickness of the shell of the central drum would severely limit the rate of transfer of heat through the shell to the web.
Heat transfer drums are described in Fleissner et al, U.S. Pat. No. 3,581,812; Kilmartin, U.S. Pat. No. 3,838,734; and Beghin, U.S. Pat. No. 4,090,553; Heisterkamp, U.S. Pat. No. 3,237,685; Cappel et al, U.S. Pat. No. 4,183,298; Appel, U.S. Pat. No. 4,252,184; Schiel, U.S. Pat. No. 4,254,561 and Wedel, U.S. Pat. No. 4,440,214. A press having a free floating high pressure nip roll is described in "HI-I Press, Mark III Installed At Scott Paper, Mobile;" Pulp and Paper Magazine of Canada, Nov. 15, 1968, pages 56-57.
The attainable speed for drying paper is often limited by the need to maintain web integrity during the forming and drying process. At high moisture contents the web is held together by water viscosity, surface tension, and the fiber contact sites. As the web is dried, the influence of viscosity and surface tension decreases both because there is less water and because viscosity and surface tension decrease with an increase in temperature; and the influence of bonding sites increases. The web will actually lose strength as it is initially heated in the dryer. This is seen in FIG. 11 which illustrates the passage of a web of paper through the forming, pressing and drying section of a paper machine and shows the change in strength characteristics of the paper web through the machine as the sheet dries. FIG. 12 is a similar figure for newsprint. It shows the breaking length and web strength characteristics of a web of newsprint as it passes through the pressing and drying operation. FIG. 12 is from Thomas, U.S. Pat. Nos. 4,359,827 and 4,359,828 and the phenomenon is discussed in detail in these patents.
There are many variables which influence the degree of drying and strengthening of the web as it passes through the first drying drum and exits from that drum. There are a number of machine variables. If a belt is used to hold the web on the drum, then the tension of the belt and the diameter of the drum are factors. If a felt is used, the permeability of the felt is a factor. If a pressure nip is used, then the pressure in the nip, the residence time in the nip and the ventilation from the nip are factors. The machine speed, the tension on the web being drawn through the machine, the temperature of the heating drum and the heat recovery rate of the drum are also factors. There are also a number of variables within the web. The freeness and permeability of the web, the compressibility of the web, the bondability of the web, the dryness or moisture content of the web as it reaches the drum, the temperature of the web, and the weight and thickness of the paper or paperboard are all factors. The tendency of the web to stick to the drum is also a factor. The limiting speed in a given situation will depend on a combination of all of the above factors. A given machine will have a maximum speed for a given web or a given web will require a certain drying capacity to achieve a given speed. The operation of the machine at a capacity below the limits influenced by these various factors is not possible.
Attempting to remove moisture from the web quickly in order to accelerate the initial heating also creates a problem. If moisture vapor in the web creates interior pressure much above constraining pressures, then the internal expansion of the vapor in the web will tend to blow the web apart.
The approximate maximum machine speeds for linerboard are shown in FIG. 13. These are examples of commercial speeds for drying paper. FIG. 13 is a plot for the drying of unbleached kraft linerboard and shows machine speed in feet per minute against grade weight in pounds per thousand square feet of web. Line 40, the dotted line, indicates the possible machine speeds versus grade weights at a constant production rate of 6.75 tons per day per inch of machine width. Line 41, the solid line, shows the actual approximate maximum commercial speed at various grade weights. These speeds corresponds to a production rate in tons/day/lineal inch of machine width of 3.6 at a grade weight of 26 lb/1000 ft.sup.2, 5.3 at 42 lb/1000 ft.sup.2, 6.8 at 69 lb/1000 ft.sup.2, and 5.1 at 90 lb/1000 ft.sup.2.
Commercial linerboard machines use 1,500-2,000 lineal circumferential feet of dryer to operate at these speeds. The dryer drum temperatures will range from 212.degree. F. to 400.degree. F. and web pressures on the drum are typically up to 1-2 lb/in.sup.2 (psi). Water removal rates are on the order of 5-7 pounds per hour per square foot of drum. For some paper grades, such as tissue, a relatively high pressure nip with the drum is made to iron the wet web onto the drum.